Paint products can be tinted by adding to them highly coloured tinting paints or, with respect to pigmentation, even more concentrated colourants (=colourant pastes). Tinting is carried out in ready-made paint products, white or coloured, called base paints. The colour and opacity of base paints depends on the amount of pigments, white pigments in particular. Base paints may be replaced by base lacquers/varnishes.
Tinting paints are normally product-specific, i.e. they are produced separately for each paint type to be tinted.The number of pigments, colours and kinds of paints is limited.
Concentrated colourant pastes can be used as a part of tinting systems. These systems permit the tinting of a broad range of various types of paint products with one series of colourants. The amount of work, number of ready products and need for storage space will be reduced, at the paint plant and distribution outlets which improves and speeds up the service.
The system involves tinting machines located at various points of the distribution chain. In a paint plant tinting takes place in relatively large batches ("in-plant"), very often according to orders received. Base paints which are delivered to the distribution warehouses and points of sale, are usually tinted directly in sales packings ("in-can").
In the latest tinting systems tinting formulae are recorded in the memory of a PC (a formula databank). The machine dispenses automatically the correct mixing proportions of colourants into the base paint according to the preselected formula. The ingredients are thoroughly mixed in a shaker and the tinted paint is ready for use. Depending on the number of colours desired, 1-10 base paints per each paint type and 5-40 colourants per one tinting system are required.
The paint products are mostly tinted in shades given as code numbers on manufacturer's own colour cards or on those colour cards based on national or international shade standards (RAL, BS, NCS etc.). It is also possible to connect the tinting machine to a colour matching system which enables tinting according to a colour sample. The number of shades is then practically infinite.
It is recommendable to use tinting systems (containing base paints, colourants, the dispensing and mixing system and software for processing colours) always when it is possible. This helps to reduce stocks, minimize costs, ensure quality and improve customer service. Some base paints, especially white bases, are sold in large amounts also untinted.
Colourants in general comprise a pigment component and a liquid medium into which the pigments are dispersed. The dispersion medium consists of at least one diluent, which is an organic solvent or water, and of a binder and/or dispersing/wetting agent. In addition to a dispersing agent, also other additives may be used.
Pigments are first mixed with the dispersion medium. Dispersion of pigments has been shown to occur in three stages:
1) wetting of particle surfaces and pores by the medium PA1 2) breakdown and dispersion of pigment particles (agglomerates and aggregates) PA1 3) wetting and stabilization of the dispersed primary particles and small agglomerates and aggregates. PA1 1) the diluent comprises at least one organic solvent (other than an aliphatic alcohol or aromatic hydrocarbon) which has a proper level of polarity and volatility, and of which 20-100% is a biodegradable ester of lactic acid and a lower alcohol, preferably ethyl or isopropyl lactate, classified as not harmful. PA1 2) 70-100% of the binder component comprises at least one relatively low molecular mass ketone aldehyde, ketone and/or aldehyde resin soluble in the afore mentioned diluent, and possibly 0-30% of another colourant binder compatible with the mentioned resin part. PA1 in tests based on dissolved organic carbon (DOC): 70% PA1 in tests based on oxygen consumption or carbon dioxide generation: 60% of the theoretical maximum.
Dispersion is carried out using equipment the performance of which is based on high shear, impact and/or cavitation forces. Various types of dispersion equipment based on batch or continuous operation may be used; impact, ball, pebble, sand, bead, roller, or colloid mills; attritors, turbo mixers, high-speed disk dispersers etc. The dispersion of pigments often depends more on the process conditions and type of equipment than on the total energy input in the dispersion process.
Dispersion medium, on the other hand, plays a major role in a dispersion process. The medium should be able to wet and stabilize pigments well and be compatible with various types of paints.
In the wetting phase the liquid dispersion medium first displaces air in the pores and on the surface of pigment agglomerates and aggregates. Factors affecting the wetting ability include for example interfacial tension on the pigment/liquid surface, angle of contact and viscosity of the liquid. For effective wetting, these values should all be low.
Through electrostatic and/or steric stabilization a layer is formed on the surface of the dispersed pigment particles preventing them from attaching to each other through flocculation or similar mechanisms. If particles remain too big they are separated through sedimentation. Finally, the consistency can be modified by adding a thickener additive to the colourant in order to retard pigment sedimentation.
The same diluents have been used in the production of colourants as are utilized as solvents in the paints to be tinted. However, binders commonly used in paints can be used in colourants only seldom, because they are not readily compatible with other binders, they often have too high viscosities and poor pigment dispersing/wetting properties. Therefore special colourant binders are used which have better properties with respect to polarity and/or solubility. In binder-free colourants only dispersing agents and diluents, such as water and organic solvents, especially glycols, are used as a medium.
Colourants often have product or shade related problems regarding stability, gloss, reproducibility, cleanness of shade, or other similar properties. To overcome these problems various additives, the most important of which are dispersing and wetting agents, are needed in addition to diluents and binders.
Glycol containing and aqueous colourant pastes, so-called universal colourants, are suitable for tinting both water- and solvent-borne decorative paint products.
Glycol colourants contain alkylene glycols, usually ethylene or propylene glycol (DE-3 839 294) as a diluent. Aqueous colourants have water (GB-2 266 310) or water and glycol (DE-1 519 254 and EP-555 950) or water and another auxiliary solvent such as diethylene glycol monoethyl ether (EP-462 557) as a diluent. Further essential components of a colourant are pigments and non-ionic and/or anionic dispersing agents. Cellulose ether (DE-1 519 254), polyethylene glycol and hydroxyethyl-ethyleneurea (EP-462 557) among others are used as a binder.
The disadvantage of glycol colourants is that they usually contain harmful components when considered from health and safety at work or environmental point of view such as ethylene glycol and dispersing agents of the type alkylphenol ethoxylates, which furthermore impair the properties of solvent-borne paints, especially gloss, drying and water resistance. Pure aqueous colourants are considerably better in terms of safety than glycol containing colourants. Their use in solvent-borne paints is however limited.
In attempts to extend the scope of application of colourants to industrial paints, universal colourants not containing water have been developed. These may contain 1) a water-soluble or dispersible binder, and 2) a diluent miscible, partially or completely, both in water and in organic solvents and 3) typically one or more non-ionic or anionic dispersing agents. The production of such universal colourants is mentioned for example in patent specification EP-507 202. Colourants in accordance with the specification contain 0.5-5% of a non-ionic dispersing agent and 20-50% of a water-miscible organic solvent.
Colourants suitable for use in solvent-borne paint products like printing inks, enamel paints, and industrial bake coatings contain for example following diluents: toluene, xylene and other aromatic hydrocarbons, n-butanol and ethylene glycol ethers or ether esters (SE-72 08650-7 and EP-448 347, examples). For example natural resins, alkyd resins, urea resins, aldehyde resins, ketone resins and acrylic resins have been used as a binder. Some of the diluents (glycol ethers, glycol ether esters, ethers etc.) mentioned in the patent specifications are considered harmful to health. Very often, solvents used in these products have an offensive odour and/or they are hazardous to health and therefore the packagings must be provided with appropriate danger classification.